Portable communication devices, such as cellular telephones, typically are required to operate over a number of different communication bands. These so called “multi-band” communication devices use one or more instances of transmit and receive circuitry to generate and amplify the transmit and receive signals. However, these communication devices usually employ a single antenna to transmit and receive the signals over the various communication bands.
The antenna in such communication devices is typically connected to the transmit and receive circuitry through switching circuitry, such as a duplexer or a diplexer, or through an isolated switch element, sometimes referred to as a “transmit/receive switch” or an “antenna switch.” The switching circuitry or the isolated switch element must effectively isolate the transmit signal from the receive signal. Isolating the transmit signal from the receive signal becomes more problematic in a multiple band communications device where the transmit frequency of one communication band might overlap with the receive frequency of a different communication band.
FIG. 1 is a schematic diagram illustrating a portion of a prior art transceiver 10 showing a blocking signal interfering with a received signal. The transceiver 10 includes an antenna 12 coupled via connection 14 to an antenna switch 16. The antenna switch 16 is coupled via connection 17 to a phase shifter 18. The phase shifter 18 is coupled via bi-directional connection 19 to a transmit filter 21 and to a receive filter 22. The antenna switch 16, a phase shifter 18, transmit filter 21, and receive filter 22 form a duplexer. The transmit filter 21 receives an amplified output of a power amplifier 25 via connection 24. The receive filter 22 delivers the receive signal via connection 27 to a low noise amplifier 28. The remainder of the transmit circuitry, the remainder of the receive circuitry and the baseband processing elements are omitted from FIG. 1 for simplicity.
The antenna switch 16 isolates the transmit signal from the receive signal. When implementing a 2G or 3G transceiver, linearity and physical size of the antenna switch are significant design factors. Linearity is usually defined by what is referred to as a third order intermodulation product, referred to as IMD3. As shown in FIG. 1, the nature of this effect is that mixing products of the TX signal with an outside blocker signal fall into the RX band, as shown using the graphical illustration 41 and specifically, the vector 46. The IMD signal may deteriorate the sensitivity of the receiver if the antenna switch 18 allows a sufficiently high IMD signal.
The largest factor in IMD performance of the antenna switch 18 is the nonlinear capacitance of the off branches of the switch. As shown in FIG. 2, the antenna switch 18 comprises a number of branches 22, 24, 26 and 28, with the number of branches dependent upon the number of frequency bands implemented in the transceiver. In this example the branches 24, 26 and 28 are “off” and the branch 22 is “on”. In this example, the branches 22, 24, 26 and 28 are implemented using field effect transistors (FETS) and the gate, source and drain connections are shown in FIG. 2. The parasitic capacitances of the off branches 24, 26 and 28 becomes more linear at more negative Vgs(Vds) voltages. This is one reason that conventional 2G/3G solutions are implemented using charge pumps. FIGS. 3A and 3B show a typical source/gate/drain layout for the schematic diagram of FIG. 2. The drain and source ohmic contacts on conventional devices occupy large areas increasing both die size and parasitic capacitances. FIG. 3A shows a typical interconnection of three single gate FET devices 32, 34 and 36 having an area on the order of 120.times.547=65640 μm2. FIG. 3B is a schematic diagram of the layout of FIG. 3A. The resistance Rlin is a resistance between the drain and source of each FET device.
Therefore, it would be desirable to have an antenna switch that provides high linearity and low loss in a small area.